System and method of perpendicularly magnetizing position information in a magnetically hard layer of a moving component

ABSTRACT

A position-sensing system magnetically senses the position of a piston rod moving with respect to a cylinder. A magnetically hard layer on the piston rod provides a recording medium. Information is perpendicularly magnetized in regions of the magnetically hard layer. These regions provide an encoding scheme for determining the position of the piston rod. Magnetic-field sensors are positioned over redundant tracks of magnetically recorded regions. Each magnetic-field sensor positioned over a given track senses the same magnetized regions while the piston rod moves with respect to the cylinder. Other magnetic-field sensors can sense ambient fields for use in performing common-mode rejection. A write head can dynamically repair damaged or erased regions detected by the magnetic-field sensors. Energized by a battery-backup power source, the magnetic-field sensors and associated circuitry can continue to track movement of the piston rod when the machinery is off.

RELATED APPLICATIONS

This application is a continuation application claiming the benefit ofthe filing date of co-pending U.S. patent application Ser. No.11/782,005, filed Jul. 24, 2007, titled “System and Method ofMagnetically Sensing Position of a Moving Component,” which is acontinuation application of U.S. Pat. No. 7,295,993, issued Aug. 21,2007, the entireties of which U.S. patent application and patent arehereby incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates generally to transducers for magnetically sensingposition of a first component relative to a second component. Moreparticularly, the invention relates to a system and method ofmagnetically sensing the position of a piston rod moving relative to acylinder based on information recorded in a magnetic film on the pistonrod.

BACKGROUND

Various industrial and mobile applications use hydraulic cylinders tocontrol the movement and position of machinery. In general, thesecylinders include a cylinder barrel within which a piston is arrangedfor reciprocating motion along an axis. A piston rod is secured at oneend to the piston. The piston rod extends out of one end of the cylinderbarrel along the axis of motion. The end of the piston rod that isexternal to the cylinder barrel is coupled directly or indirectly to amachine component. The piston divides the cylinder barrel into separatechambers. Fluid entering one of the chambers causes the piston and,thus, the piston rod to move relative to the housing. This movement ofthe piston rod drives the movement of the machine component.

Precise control of the position of the piston is generally fundamentalto controlling the operation of the machinery. Measuring the position orvelocity of the piston relative to the cylinder is often needed toachieve such control using conventional feedback control techniques.Accordingly, industry has produced various mechanical, magnetic,acoustic, and optical techniques for detecting the instantaneousposition of the moving piston or piston rod.

Many position detection systems are expensive, cumbersome, or difficultto mount on the cylinder. Further, position detection systems forhydraulic cylinders often operate in harsh environments caused byinternal conditions, such as pressurized fluid that drives the motion ofthe piston, and external conditions, such as dust and debris. Some typesof position detection systems, such as Linear Variable DifferentialTransformers (LVDTs) and linear scales, can be unreliable or easilydamaged in a harsh environment.

Some techniques entail encoding piston rod positions on the piston roditself, and reading the encoded positions as the piston rod moves past areference point, using a reading technique, e.g., optical, magnetic,mechanical, suited to the particular type of encoding. Some knowntechniques cut grooves, etch recesses, or marks in the rod. Suchmodifications, however, can adversely affect the rod's strength. Anotherknown technique, described in the UK Patent Application No. GB 2 096421, is to encode the position information magnetically in the rodmaterial of the piston rod. In this UK patent application, the pistonrod is constructed of steel and can be magnetized. However, this rodmaterial is magnetically “soft.” Magnetically soft material has lowcoercivity, which is a measure of difficulty for magnetically encodingand erasing information in that material. Thus, the position informationencoded in rod material with low coercivity is subject to accidentalerasure or alteration.

SUMMARY

In one aspect, the invention features a position-sensing system,comprising a cylinder, a piston rod movably coupled to the cylinder formovement with respect thereto, and a magnetically hard layer formed onthe piston rod to provide a recording medium. A plurality of regions ofthe magnetically hard layer is perpendicularly magnetized to provide anencoding scheme for determining a position of the piston rod relative tothe cylinder. A plurality of magnetic-field sensors is coupled to thecylinder in proximity of the magnetically hard layer to sense theperpendicularly magnetized regions while the piston rod is moving withrespect to the cylinder. At least two of the magnetic-field sensorsbeing axially positioned to sense the same set of magnetized regions insuccession and to generate signals in response to the sensed magnetizedregions that can be used to determine a position of the piston rod.

In another aspect, the invention features a method for sensing aposition of a piston rod moving relative to a cylinder. The methodcomprises forming a magnetically hard layer on the piston rod to providea recording medium for storing information. Information isperpendicularly magnetized at a plurality of regions of the magneticallyhard layer. A plurality of magnetic field sensors read the sameperpendicularly magnetized regions of the magnetically hard layer whilethe piston rod is moving relative to the cylinder. Output signals aregenerated by the plurality of magnetic field sensors from which aposition of the piston rod with respect to the cylinder can bedetermined.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of this invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which like numerals indicate likestructural elements and features in various figures. The drawings arenot necessarily to scale, emphasis instead being placed uponillustrating the principles of the invention.

FIG. 1 is a cross-section view of an embodiment of a cylinder, includinga piston, a piston rod, a magnetically hard layer on the piston rod, anda flux-sensing apparatus adjacent the magnetically hard layer.

FIG. 2 is a cross-section view of the cylinder illustrating oneembodiment of the flux-sensing apparatus having a plurality of spatiallyseparated read sensors for reading information magnetically recorded inthe magnetically hard layer.

FIG. 3 is a cross-section view of the cylinder illustrating anotherembodiment of the flux-sensing apparatus having redundant tracks ofspatially separated read sensors.

FIG. 4 is a diagrammatic top view of an embodiment of a flux-sensingapparatus with a plurality of serially positioned read sensors and aradially positioned read sensor for sensing rotational movement of thepiston rod.

FIG. 5 is a diagrammatic side view of still another embodiment of theflux-sensing apparatus, including a read sensor for sensing ambientfields, the read sensor being disposed at a greater distance from themagnetically hard layer than the spatially separated read sensors thatread the information magnetically recorded in the magnetically hardlayer.

FIG. 6 is a diagrammatic top view of an embodiment of a flux-sensingapparatus with a read sensor for reading information magneticallyrecorded in the magnetically hard layer and flux concentrator forcollecting and directing flux to the read sensor.

FIG. 7A and FIG. 7B are simplistic diagrammatic views of two techniquesfor magnetically recording information in the magnetically hard layer onthe piston rod.

FIGS. 8A-8D are cross-sectional views of layer geometries for differentembodiments of piston rod construction, each embodiment having amagnetic film that provides a recording medium for magneticallyrecording information.

FIG. 9 is a diagram of an embodiment of a write head for magneticallyrecording information in the magnetically hard layer.

FIG. 10A is a diagram of a rod coated with a magnetically hard layerhaving a plurality of magnetized regions that provide a relativeencoding scheme for use in determining the position of the rod.

FIG. 10B is a diagram of a rod with a magnetically hard layer having aplurality of magnetized rings around the circumference of the rod thatprovide a relative encoding scheme for use in determining the positionof the rod.

FIG. 10C is a diagram of a rod with a magnetically hard layer havingregions encoded to provide an absolute encoding scheme in combinationwith a relative encoding scheme.

FIG. 10D is a diagram of a rod with a magnetically hard layer havingregions encoded in a checkerboard pattern to provide axial and radialmagnetic transitions along the rod.

DETAILED DESCRIPTION

In general, the present invention features a position-sensing system formagnetically sensing position, distance traveled, velocity,acceleration, and direction of motion of a first component with respectto a second component. In general, components are parts or elements of asystem or of an assembly, examples of which include, but are not limitedto, machines, equipment (e.g., backhoes), vehicles (e.g., tractors),tools, and conveyors. Systems that can embody the invention include, butare not limited to, hydraulic systems, pneumatic systems, vibration andshock damper systems, and measurement systems for precision machinery.Various vehicular or machine data buses can employ the invention toprovide open-loop sensing, closed-loop feedback system control, orcombinations thereof. Other types of positioning systems, such as GPS(Global Positioning Systems), can integrate functionality for detectingthe location and distance of a machine with the functionality of theposition-sensing system for detecting and controlling the machine'smotion.

In a specific embodiment, the position-sensing system includes a pistonrod (or cylinder rod) that moves relative to an actuating cylinder. Inaccordance with the principles of the invention, a physically andmagnetically hard material coats a portion of the piston rod. With theuse of standard magnetic recording techniques, information is recordedin this coating layer (or film) of magnetically hard material in theform of magnetic bits, dots, or marks (also referred to generally as amagnetic encoding). This position-sensing system uses this recordedinformation to determine the position of the piston rod. As used herein,magnetically “hard” material is material with high coercivity. Magneticmaterial of high coercivity requires considerable energy to magnetize,i.e., record, information, but also to demagnetize recorded information.This magnetization of the encoding within the magnetically hard layercan occur longitudinally or perpendicularly. In one embodiment,longitudinally magnetized encoding is employed without there being anon-magnetic layer between the magnetically hard layer and the pistonrod.

One or more flux-sensitive magnetic read heads or sensors, mounted in,on, or near an end cap of the cylinder, sense the magnetic encodingwhile the piston rod moves past. Circuitry in communication with theread sensors can process and store the information obtained from themagnetic encoding to determine the instantaneous incremental position ofthe piston rod, its velocity, acceleration, and direction of motion(i.e., linear, rotational, or both) relative to the cylinder. Inaddition, the circuitry can produce a signal, representing any of suchpositional information, for display or for use in controlling positionor movement of the machine or of a component thereof.

Various encoding techniques use relative encoding to record positioninformation in the magnetically hard layer on the piston rod. One suchtechnique includes evenly spaced marks magnetically recorded in a trackor column that extends axially, i.e., along a length, of the piston rod.A read sensor detects magnetic transitions between adjacent magneticmarks. With reference to a zero position, circuitry cumulatively countsmagnetic transitions detected by a read sensor and remembers the linear(and/or rotational) position of the piston rod based on the count.Magnetic patterns other than evenly spaced marks can also reside in themagnetically hard layer, in combination with the evenly spaced marks,for auxiliary functions, e.g., for signifying a key position on thepiston rod or for identifying a particular event.

In one embodiment, two or more spatially separated serial read sensorsread the magnetic marks of one track. Flux concentrators near the readsensors can improve the sensing of the flux of the magnetic bits. Fromphase differences in signal outputs produced by the read sensors, thecircuitry can identify the linear and rotational position, traveleddistance, velocity, acceleration, and direction of motion of the pistonrod. Other read sensors can be used to sense ambient flux for use incommon-mode rejection computations. When power to a machine employingthe invention is off, the read sensors and read-head electronicscircuitry can continue to receive power from an auxiliary power supply(e.g., a battery). Consequently, the read sensors can continue to detectmotion if the piston rod drifts, and the circuitry can continue tocompute the piston rod position, while the machine is off. Then, whenthe machine begins to operate again, the circuitry knows the currentposition of the piston rod without any need of calibration.

To protect against accidental or intentional erasure of informationrecorded in the magnetically hard layer, the position-sensing system ofthe invention can employ redundancy in the form of evenly sizedmagnetized rings that each encircle the piston rod or in the form ofduplicate longitudinal tracks or columns extending axially on the pistonrod. Redundant read sensors disposed about the circumference of thepiston rod can each sense the rings or marks of a different track. Avoting mechanism can determine which read sensor or sensors aredetecting valid information and which information to use. In oneembodiment, the position-sensing system integrates a write-head with theread sensors so that if a read sensor detects an erased or failingmagnetic ring or mark, the write-head can dynamically repair theencoding.

Although herein described primarily with respect to cylinders and pistonrods, practice of the invention can involve various other types ofcomponents. In general, the invention can be embodied by any twosurfaces that move sufficiently near each other so that magnetic-fieldsensors on one surface can detect a magnetic recording in a magneticallyhard layer on the other surface. For example, the components cancomprise two planar surfaces, with implementation of the magneticallyhard layer occurring on a first planar surface and the read sensorsoccurring on a second planar surface. As another example, the componentsembodying the position-sensing system of the invention can include amachine component that is actuated by the piston rod. The position,direction of movement, distance traveled, velocity, or acceleration ofthe actuated machine component can be correlated to the position,direction of movement, distance traveled, velocity, or acceleration ofthe actuator (i.e., the piston rod).

FIG. 1 shows a cross-sectional side view of an embodiment ofposition-sensing system having a cylinder 2, including a cylinder barrel3, a cylinder end cap 4 (also called a “packing gland”), and a housing6. A piston 8 is arranged within the cylinder barrel 3 for reciprocatingmotion along an axis 9. The piston 8 partitions the cylinder barrel 3into two chambers 10 a and 10 b. One end of a piston rod 12 is securedto the piston 8. The piston rod 12 extends along the axis 9 of motion.The other end of piston rod 12 extends out of the housing 6 through theend cap 4, and may be coupled directly or indirectly to a machinecomponent. Typically, the piston 8 and piston rod 12 are constructed ofa ferromagnetic material (e.g., steel), although a non-magnetic pistonrod can be used to practice the invention. In the embodiment shown, thepiston rod 12 is cylindrical; other piston rod shapes can be employedwithout departing from the principles of the invention.

A magnetically hard film or layer 14 coats the piston rod 12 to providea recording medium. This coating can be continuous or discontinuous onthe piston rod 12 and cover a portion or all of the piston rod 12. Forexample, typically the magnetically hard layer 14 is not formed on theend of the piston rod 12. Regions of the magnetically hard layer 14along a length of the piston rod 12 are magnetized. Each magnetizedregion is an area of aligned magnetic dipoles. Such magnetized regionsmay be referred to herein, individually or collectively, as magneticbits, dots, marks, patterns, or encoding. The magnetized regions areused to determine a position of the piston rod 12 with respect to thecylinder 2, as described in more detail below.

The end cap 4 has a channel 16 for the passage of fluid (e.g., oil,water, steam, gas) into and out of the chamber 10 b, for moving thepiston 8. A fluid passageway to the other chamber 10 a is not shown.Seals 18 within the end cap 4 are arranged to lie flush with a surfaceof the piston rod 12 and thus prevent fluid from leaving the chamber 10b.

The housing 6 encloses a flux-sensing apparatus 20—comprising one ormore read heads or sensors—and read-head electronics 22. The readsensors of the flux-sensing apparatus 20, also referred to asmagnetic-field sensors, can be arranged to sense a magnetic fieldgradient axially along the surface of the piston rod and radially alongthe radius of the piston rod. Each read sensor of the flux-sensingapparatus 20 can be, for example, a Hall-effect device, a coil, amagnetoresistive (MR) sensor, or a giant magnetoresistive (GMR) sensor,such as a NVE AB-002-00 read sensor produced by NVE Corporation of EdenPrairie, Minn. GMR sensors, for example, are available in dual-inlinepackages (DIP) that are approximately 3 mm by 3 mm by 1 mm in size).These read sensors can remotely sense the magnetized regions of themagnetically hard layer through non-magnetic layers, whether metallic,such as chrome, or non-metallic, such as plastic.

The location of the flux-sensing apparatus 20 within the housing 6provides protection from the environment and permits ready access foreasy replacement (i.e., the housing 6 can be removed without removingthe end cap 4 from the cylinder 2). The flux-sensing apparatus 20 ismounted in the housing 6 within proximity of the piston rod's surface topermit sensing of the encoding recorded in the magnetically hard layer14. Machining away part of the DIP housing of a read sensor, orpackaging the read sensor in a package type other than a DIP, can moreclosely position the read sensor to the magnetically hard layer 14 fordetecting greater amounts of flux from the magnetic encoding. Althoughshown in FIG. 1 to be on only one side of the piston rod 12, theflux-sensing apparatus 20 can encircle the piston rod 12 within thehousing 6. The housing 6 also includes a rod wiper 24 for wiping awaysmall magnetizable particles that may adhere to the piston rod 12. Inanother embodiment, the end cap 4 houses the flux-sensing apparatus 20and read head electronics 22. In such an embodiment, the housing 6 isoptional because the end cap 4 can protect the read head 20 from theharsh operating environment.

In brief overview, fluid within the chambers 10 a, 10 b at time-varying,differential pressures causes the piston 8 and thus the piston rod 12 tomove in and out relative to the flux-sensing apparatus 20. Theflux-sensing apparatus 20 reads the recorded magnetic encoding on thepiston rod 12 and produces corresponding analog or digital signals. Fromthese signals, the read-head electronics 22 can determine and store theposition, distance traveled, velocity, acceleration, and direction ofmotion of the piston rod, or any combination thereof. In addition, theread-head electronics 22 can produce a signal, representing any of suchpositional information, for display or for use in controlling positionor movement of the machine or a component thereof.

FIG. 2 shows the housing end of one embodiment of a position-sensingsystem, in which the flex-sensing apparatus 20 includes a plurality ofread sensors 21-1, 21-2 (generally, 21) positioned serially to read thesame portion (i.e., track) of magnetically recorded regions in themagnetically hard material 14, while the piston rod 12 moves linearly inand out of the cylinder 2. The read sensors 21 are disposed axially andseparated by a known distance (e.g., 0.5 mm). From signals produced bythe read sensors 21, the read-head electronics 22 can compute the axialdistance of the piston rod 12 independently of the absolute value of thesignals. Such computations are based on the ratio between the signalsproduced by each read sensor 21. For example, if the spacing betweenmagnetic transitions on the piston rod is 1 mm, each read sensor 21produces a sinusoidal output signal having a peak amplitude every 1 mm.Because of the spacing between adjacent read sensors, the sinusoidaloutput signals produced by such read sensors have a separation in phasecorresponding to their spatial separation. From the ratio of theseoutput signals, the read-head electronics 22 can establish the linearposition of the piston rod at finer resolution than 1 mm. In addition,from the phase differences in the sinusoidal output signal, theread-head electronics 22 can also determine the rotational position ofthe piston rod 12. Although FIG. 2 shows only two read sensors 21-1,21-2, it is to be understood that more than two serially positioned readsensors can be used in the practice of the invention.

FIG. 3 shows another embodiment of the flex-sensing apparatus 20′ thatcan be used in the position-sensing system of FIG. 1. The flex-sensingapparatus 20′ includes sets 23-1, 23-2 (generally 23) of seriallypositioned, axially spaced-apart read sensors 21-1, 21-2 (generally 21).Here, the sets 23 are on opposite sides of the piston rod 12 (i.e.,separated by 180°). In other embodiments, there can be more than twosets 23 of read sensors. In addition, the separation between the sets 23of read sensors 21 can be other than 180°.

Depending upon the pattern encoded in the magnetically hard layer, theseparate sets 23 of read sensors 21 can sense the same magnetizedregions (from different vantage points) or different magnetized regionsthat have been magnetically recorded with identical and, thus, redundantinformation. The read-head electronics 22 can employ a voting orelection mechanism between (or, for more than two, among) correspondingread sensors 21, i.e., those read sensors concurrently sensing the samemagnetized region or redundant magnetized regions. The voting orelection mechanism determines which output signals from the read sensors21 to use for determining the current position of the piston rod 12. Asan example, the mechanism can average the output signals of the readsensors, discarding any reading clearly deviating from valid values.Accordingly, the read-head electronics 22 can also determine whichmagnetized regions have become erased or damaged and, if integrated witha write head, as described below, cause re-magnetization of suchregions. The additional tracks of read sensors provide redundancy,reliability, and resistance to damage.

FIG. 4 shows a top view of another embodiment of the flux-sensingapparatus 20″ including the plurality of serially positioned readsensors 21-1, 21-2 (FIG. 2) and another radially positioned read sensor21-3. In this exemplary embodiment, the read sensor 21-3 is positionedadjacent the read sensor 21-1, separated from the read sensor 21-1 by aknown distance (e.g., 0.5 mm). As shown, the serially positioned readsensors 21-1, 21-2 are positioned over a track 28 of magnetized regions30. The read sensor 21-3, which is at a same distance from themagnetically hard layer 14 as the other read sensors 21-1, 21-2, ispositioned alongside the track 28. Other embodiments can position theread sensor 21-3, for example, adjacent to the other read sensor 21-2 orequidistant to the read sensors 21-1, 21-2. Still other embodiments canhave another radially positioned read sensor, positioned adjacent to theread sensor 21-2 and serially aligned with the first radially positionedread sensor 21-3.

The embodiment shown in FIG. 4 is particularly suited for sensingrotational movement of the piston rod 12, while the piston rod 12 movesin the direction indicated by the arrow 32 relative to the flux-sensingapparatus 20″. The read-head electronics 22 (FIG. 1) computes linearposition and rotational position of the piston rod 12 based on theoutput signals of each of the read sensors 21-1, 21-2, and 21-3. If thepiston rod 12 rotates, the read sensor 21-3 senses the magnetizedregions 30 more strongly, or more weakly, depending upon the directionof rotation. The read-head electronics 22 can determine the degree ofrotation based on the known distance of the read sensor 21-3 from theread sensor 21-1 and on the ratio of their output signals.

Common-Mode Rejection

The ferromagnetic substrate of the piston rod 12 below the magneticallyhard layer 14 may become magnetized and generate a field that interfereswith the magnetic fields of the regions magnetically recorded in themagnetically hard layer 14. One embodiment of the position-sensingsystem uses common-mode rejection (CMR) to negate the effect of ambientfields on those read sensors 21 reading the magnetized regions. FIG. 5diagrammatically shows an example of a flex-sensing apparatus 20′″, inwhich a CMR read sensor 21′ is positioned at a greater distance from themagnetically hard layer 14 than the read sensors 21-1, 21-2. The CMRread sensor 21′ is, in one embodiment, approximately ten times thedistance of the primary read sensors 21-1, 21-2 from the piston rod,whereas the read sensors 21-1, 21-2 are at a distance 29 near enough tothe magnetically hard layer 14 to sense the magnetic fields of themagnetized regions 30 while the piston rod 12 moves by in the directionsindicated by double-headed arrow 32.

Being far enough from the magnetically hard layer 14 not to sensestrongly the magnetic fields of the magnetized regions 30, the CMR readsensor 21′ predominantly senses ambient fields. The read-headelectronics 22 uses the signals produced by the CMR read sensor 21′ tocancel out interference from an ambient field upon the magnetic fieldssensed by the read sensors 21-1, 21-2. Thus, the common-mode rejectionof magnetic fields produced by a magnetization of the ferromagneticsubstrate enables high resistance to interfering ambient fields.Although only one CMR read sensor 21′ is shown, the invention can bepracticed with multiple such CMR read sensors. For example, oneembodiment has a CMR read sensor 21′—used for sensing an ambientfield—for each read sensor 21 used to sense the magnetically recordedregions.

Flux Concentrator

FIG. 6 shows a top view of an embodiment of the flux-sensing apparatus20″″ in which a flux concentrator 34 is fabricated on each side of oraround each read sensor 21-1, 21-2, to enhance the sensing of themagnetized regions 30. Made of magnetically soft, i.e., low coercivity,material, each flux concentrator 34 collects the flux of the magnetizedregion 30 from around the circumference of the piston rod 12 and directsthe collected flux to the read sensor 21 with which that fluxconcentrator 34 is associated. Collecting flux from the circumference ofthe piston rod 12 makes each read sensor 21 insensitive to the angularposition of the read sensor 21 relative to the magnetized region 30 andboosts the signal. An advantage stemming from the use of fluxconcentrators is that the construction of the magnetically hard layer 14can be thinner than would be otherwise. In addition, the stronger themagnetic signal that a read sensor can derive from the magnetically hardlayer 14, the farther away the read sensor can be from the magneticallyhard layer.

FIG. 7A and FIG. 7B are diagrammatic views of two standard magneticrecording techniques for magnetically recording information in themagnetically hard layer 14 of FIG. 1. A write transducer (i.e., a polewrite head) can magnetize the magnetically hard layer 14 with anidentifiable magnetic pattern in one of two standard ways: 1)longitudinal; and 2) perpendicular. When a current is applied to thewrite transducer, an external field is generated, thereby aligning themagnetic domains within the magnetically hard layer 14. Writetransducers are currently able to record on the order of tens ofmegabits per square inch.

In longitudinal media, the magnetization lies in the plane of themagnetically hard layer 14, which is shown in FIG. 7A as left and rightarrows. The magnetic pattern of longitudinal recording media consists of“transitions,” i.e., reversals of the in-plane magnetization from onepolarity to the other. Such a reversal is marked by the existence ofmagnetic poles whose stray flux is sensed by a read head located abovethe medium. In perpendicular media, the magnetization is perpendicularto the plane, shown as up and down arrows in FIG. 7B. Here, the magneticmarking occurs by creating transitions between regions magnetized “up”and “down.”

Longitudinal and perpendicular recording media can both be produced byelectrochemical methods (e.g., electroless plating, electroplating,chemical vapor deposition, and electrochemical deposition (sputtering))or by means of adhesive layers or strips of magnetic tape. Forlongitudinal and perpendicular recording media, the materials used areoften cobalt-based alloys. Pure cobalt (Co) can be used to produce amagnetic film of high coercivity, but alloying elements are typicallyused to tailor the magnetic properties of the recording media and toincrease its coercivity. Examples of alloying elements include group VA(P, As, Sb, Bi), group VIB (Cr, Mo, W) elements, and the noble elementsPt and Pd. For longitudinal media, example alloys include Co—P, Co—Ni—P,Co—W, and Co—Mo. For perpendicular media, example alloys include Co—P,Co—W, and Co—Cr. Approximate high coercivity values obtained from usingsuch Co-based alloys range from 1-2 kOe. Embodiments of the magneticallyhard layer 14 can be synthesized with such materials to produce amagnetic layer with high coercivity. In a preferred embodiment, themagnetically hard layer 14 is made of a CoNiP alloy. With a CoNiP alloy,the magnetically hard layer 14 can be synthesized for eitherperpendicular or longitudinal recording.

For longitudinal media, for instance, a ferromagnetic substrate can posea problem for information retention. Being magnetically permeable, theferromagnetic substrate provides a low reluctance path for the flux.Consequently, longitudinal recording can exhibit a reduced level ofmagnetic flux to be sensed and has generally required the use of anon-magnetic layer between the magnetically hard layer 14 and theferromagnetic piston rod 12 to prevent the loss of the available flux. Anon-magnetic alloy, for example, such as nickel-phosphorous (NiP),disposed between the piston rod 12 and the magnetically hard layer 14,can improve the amount of flux available for sensing. The use of CoNiPfor the magnetically hard layer 14 of the invention, however, enableslongitudinal recording without the need of such an interveningnon-magnetic layer, although use of the non-magnetic layer is stilladvantageous.

Magnetic layers or films of high coercivity, such as the magneticallyhard layer 14, can maintain recorded information under external strayfields and under the demagnetization fields deriving from the imposedmagnetic transitions. This magnetically hard layer 14 may also providegood mechanical and corrosion resistance. However, considering the harshoperational environment of the cylinder 2, the magnetically hard layer14 can be coated to insure sufficient resistance to mechanical wear andcorrosion. One example of such a protective layer can be hard chrome,i.e., a Cr layer. Notably, a chrome protective layer exhibits strongadhesion to a CoNiP magnetically hard layer 14 (similar to the strengthof adhesion of chrome to steel).

FIGS. 8A-8D show cross-sectional views of various embodiments oflayering geometries, including a substrate 50 (e.g., the piston rod), anoptional intermediate layer 52, the magnetically hard layer 14, and anoptional protective layer 54. The layers 14, 52, and 54 can cover all ora portion of the piston rod 12. For example, the magnetically hard layer14 (and the optional layers 52, 54) are not typically applied to the endof the piston rod 12, in particular, to those one or more portions ofthe piston rod 12 that do not pass near enough to a read head forsensing.

Generally, the substrate 50 can be magnetic or non-magnetic, e.g.,although typically ferromagnetic (e.g., a steel rod), the piston rod 12can be constructed of non-magnetic material (e.g., plastic, aluminum,ceramic, or glass) without departing from the principles of theinvention. In each of the embodiments, the recording media (i.e., themagnetically hard layer 14) is comprised a high coercivity magneticmaterial, such as CoNiP, and the optional protective layer 54 is made ofchrome. The composition of the optional intermediate layer 52 dependsupon the type of the recording media and of the substrate material.

FIG. 8A shows an embodiment in which the magnetically hard layer 14 isproduced as longitudinal media and the substrate material isferromagnetic. Here, the intermediate layer 52 can be a non-magneticamorphous layer, such as Ni—P, to obstruct the low reluctance paththrough the substrate 50 and, consequently, to improve the magnitude ofthe flux being sensed and retention of the signal after being exposed toa strong magnetic field (e.g., from a permanent magnet). As describedabove, use of a non-magnetic intermediate layer 52 is optional when themagnetically hard layer 14 has a composition, such as CoNiP describedabove, that enables sufficient flux to be sensed despite any shunting offlux by the substrate 50.

FIG. 8B shows an embodiment in which the substrate material isnon-magnetic and the magnetically hard layer 14 serves as longitudinalmedia. Non-magnetic substrates lack the permeability of ferromagneticsubstrates, and consequently have less need for a non-magneticintermediate layer 52 than ferromagnetic substrates. Notwithstanding, anon-magnetic intermediate layer 52 can be used, with the advantagesnoted above.

FIG. 8C shows an embodiment in which the magnetically hard layer 14provides perpendicular media and the substrate is made of aferromagnetic material. For perpendicular media, the permeability of aferromagnetic substrate serves an advantage. The return path in aferromagnetic substrate between adjacent oppositely magnetized regionsdoes not affect the stray flux in the region above the magnetically hardlayer 14, and assists in the write process and in the retention of thewritten information. Here, too, the use of an intermediate layer 52 isoptional, although a magnetically soft intermediate layer 52 can maskunreliable or non-uniform permeability of the ferromagnetic substrate50, and therefore its presence can be beneficial.

FIG. 8D shows an embodiment in which the magnetically hard layer 14provides perpendicular media and the substrate 50 is made of anon-magnetic material. Non-magnetic substrates lack the beneficialproperties of permeability for information retention. To improve theinformation retention of perpendicular media on non-magnetic substrates,the intermediate layer 52 can be constructed as a magnetically softlayer (e.g., permalloy or Ni—Fe).

Each of the FIGS. 8A-8D also shows the relative thicknesses of thelayers 14, 52, and 54 on the substrate 50 of the piston rod 12. In eachillustrated embodiment, the magnetically hard layer 14 is approximately5 um thick, the optional intermediate layer 52, when present, isapproximately 1-10 um thick, and the optional protective layer 54, whenpresent, is approximately 40 um thick. The thickness of the protectivelayer 54 affects the resolution of the piston rod position sensingsystem by limiting how near the read heads can be to the magneticallyhard layer 14. For example, with a 40 um thick protective layer 54,magnetically recorded marks may need to be spaced apart by at least 40um (approximately) for the read heads to be able to distinguish betweenthem. In embodiments without the protective layer 54, the marks can belocated more closely together because the read heads can be nearer tothe encoded magnetically hard layer 14. The particular thicknesses shownin FIGS. 8A-8D provide illustrative examples; other thicknesses for thelayers 14, 52, and 54 can be used to practice the invention.

FIG. 9 shows diagrammatically a process in which a write head 80 writesa series 84 of regions 88 to the magnetically hard layer 14 along alength of the substrate 50. Magnetically recording the encoding in themagnetically hard layer 14 can occur before installation of the cylinder2 into the particular machinery or during operation of the cylinder 2,as described further below. In one embodiment, the write head 80 is ofthe kind typically used for magnetic tape recording, which is alongitudinal recording medium. World Magnetics, Inc. of Traverse City,Mich.) produces write heads that can be used to practice the invention.An electrical current flowing through a coil 92 generates a magneticfield in the write head 80. The magnetic field is strongest in the gapabove the magnetically hard layer 14 and induces the magnetic domains inthe magnetically hard layer 14 to align themselves as the write headmoves relative to the substrate 50. Arrow 96 represents the direction ofmotion of the write head 80 relative to the substrate 50. Each region 88is made of many magnetic domains, oriented as a group by the magneticfield from the write head 80. In one embodiment, the write head 80 isenergized with 0.75 amperes of current to generate magnetized regions88.

Position information can be recorded in the magnetically hard layer 14of the piston rod 12 in a multitude of ways. Some techniques explicitlyrecord the identities of the absolute piston rod positions on the pistonrod 12 (e.g., using binary code), other techniques magnetize shapes orregions of the magnetically hard layer 14 from which piston rodpositions can be computed, and still others use relative positiontechniques to determine the current location of the piston rod. Examplesof techniques for recording absolute rod positions and for magnetizingshapes are described in U.S. patent application Ser. No. 10/840,781,filed May 6, 2004, titled “Systems and Methods of Recording Piston RodPosition Information in a Magnetic Layer on a Piston Rod,” the entiretyof which is incorporated by reference herein.

FIGS. 10A-10D illustrate different examples of patterns of magnetizedregions 120 that implement relative encoding schemes. In each of theseexamples, the magnetized regions 120 are represented as beinglongitudinally recorded, i.e., in the plane of the magnetically hardlayer 14, although it is to be understood that perpendicular recordingcan be used to practice the invention. FIG. 10A shows a plurality ofredundant tracks 124 of identically sized magnetized regions 120extending axially along a length of the piston rod 12. In oneembodiment, each magnetized region 120 is approximately 1 mm in lengthand 1 mm in width. Within a given track 124, magnetization of themagnetized regions 120 occurs in an alternating pattern, i.e., everyother magnetized region 120 has the same magnetic alignment. Formagnetized regions 120 of 1 mm length, this alternating pattern producesa transition (i.e., polarity reversal) between neighboring regions 120every 1 mm. The tracks are redundant in that the information that can beobtained from one given track is identical to the information that canbe obtained from any of the other tracks: the tracks 124 are equal toeach other in length, start at the same distance from one end of thepiston rod 12, and have magnetic transitions at equal distances alongthe piston rod 12 so that read sensors aligned circumferentially aroundthe piston rod can detect the transitions simultaneously. In addition,these tracks 124 can be evenly spaced around the circumference of thepiston rod 12. For example, four tracks 124 of magnetized regions 120evenly spaced around the circumference of the piston rod are 90 degreesapart.

Referring also to FIG. 3, one set 23 of read sensors 21-1, 21-2 isaligned over each track 124 and, while the piston rod 12 moves, eachread sensor 21 senses the magnetized regions 120 of one of the tracks124 and produces a corresponding sinusoidal output signal. The peak ofeach sinusoidal output signal occurs over the magnetic transitionbetween adjacent magnetized regions 120, and the trough occursapproximately over the midsection of the each magnetized region.Accordingly, each set 23 of read sensors produces a plurality ofsinusoidal output signals separated in phase, as described above. Theredundant information of the tracks 124 should result in output signalsof one set 23 of read sensors being similar in phase and amplitude tothose output signals produced by each of the other sets 23 of readsensors (provided no magnetically recorded information in any of thetracks 124 is damaged or erased).

From the output signals, the read-head electronics 22 maintains a countof the sensed magnetized regions 120 (i.e., the transitions). Asdescribed above, the read-head electronics 22 can use an electionmechanism to determine from the output signals produced by the sets 23of read sensors that a transition has been sensed. The count canincrement and decrement based on the current direction of oscillatingmotion of the piston rod 12. With reference to a designated zero-pointon the piston rod 12, the read-head electronics 22 determines theabsolute position of the piston rod from the current count. From theretained count of magnetized regions, the read-head electronics 22 canalso determine distance traveled, speed, direction and acceleration ofthe piston rod. Although the computation of these parameter values canbe obtained from a single track 124, the use of redundant tracks 124increases the amount of information stored in the magnetically hardlayer 14, and improves the reliability of measurement and robustness ofthe position-sensing system. In addition, the read-head electronics 22can maintain a plurality of counts, e.g., one count for each readsensor, and another count that increments only, to maintain a cumulativecount of distance traveled by the piston rod, etc.

Rotational movement of the piston rod 12 is another parameter that theposition-sensing system can detect and monitor. In one embodiment, theread-head electronics 22 determines from the output signals the degreeof rotation with respect to a designated zero-point position. In thisembodiment, a count maintained by the read-head electronics correspondsto the extent the piston rod 12 has rotated within the cylinder 2 withrespect to this zero-point position. The number of sets 23 of readsensors determines a degree of resolution for measuring the rotationalposition of the piston rod 12: the greater the number of read sensorsets, the finer the resolution. There may be more sets 23 of readsensors than tracks 124 of magnetized regions 120.

FIG. 10B shows another embodiment in which identically magnetized rings120′ encircle the circumference of the piston rod 12. The thickness(i.e., axial length) of each ring satisfies the required resolution forsensing. Similar to the tracks 124 of FIG. 10A, magnetization of therings occurs in an alternating pattern, i.e., every other magnetizedregion 120 has the same magnetic alignment. A plurality of sets 23 ofread sensors located around the piston rod 12 in the housing, asdescribed above in connection with FIG. 3, can simultaneously sense eachmagnetized ring 120′. An advantage of magnetized rings is that thepiston rod 12 can rotate without affecting the ability of the readsensors 21 to sense the linear position of the piston rod; that is,there is no dependence on precise alignment between the read sensors 21and tracks of magnetized regions. In effect, this embodiment does nothave tracks in the sense of FIG. 10A. In this embodiment, the redundantsets 23 of read sensors 21, not the magnetized regions 120′ definedistinct tracks.

FIG. 10C shows another embodiment in which certain types of magnetizedregions 132 are interspersed with the magnetized regions 120. Thespecial magnetized regions 132 can serve to identify a particularlocation on the piston rod (i.e., identify an absolute position) or toprovide an event marker. In one embodiment, these special magneticregions 132 have twice the axial length of the other magnetized regions120 (i.e., comprised of two adjacent regions 120 magnetized with thesame magnetic alignment). When the read sensors 21 detect these magneticregions 132, the read-head electronics 22 determines that the piston rodhas reached a particular position within the cylinder 2. For example,such magnetized regions 132 (i.e., double-length regions) can appearafter every ten magnetized regions 120. The means for distinguishingspecial regions 132 from the other magnetized regions 120 may varywithout departing from the principles of the invention (e.g., thrice thelength of the magnetized regions 120, or twice the width of themagnetized region 120, or combinations of different length and widths).

FIG. 10D shows another embodiment in which the magnetized regions 120are recorded in the magnetically hard layer 12 in a checkerboardpattern: each track 124 has an alternating pattern of positive andnegative polarities and every other track 124 has the same alternatingpattern. Accordingly, each magnetized region 120, other than those atthe upper and lower edges of the pattern, is surrounded on four sides bya magnetized region 120 recorded with an opposite polarity. Thus, a readsensor 21 can sense a magnetic transition when the piston rod 120 moveslinearly and rotationally.

As noted above, relative encoding schemes determine piston rod positionby maintaining a count of sensed magnetized regions. The count needs tobe remembered even after the machine is turned off. In some types ofmachinery, the piston rod 12 may drift after the machine is turned off(e.g., after the hydraulics bleed off). Accordingly, a batterycontinually energizes the read sensors 21 and read-head electronics 22when power to the machine is off. By maintaining a supply of power, theread sensors 21 can detect any drifting movement of the piston rod andthe read-head electronics 22 can compute and record the piston rodposition although the machine is not operating. When the machinery isturned on again, the current position of the piston rod within thecylinder is known.

Maintenance

In one embodiment, a write head can be integrated with a read sensor,e.g., in the flux-sensing apparatus 20 of FIG. 1, to allow dynamicrepair of damaged or erased magnetic bits. The write head can be axiallyaligned with and spatially separated by a known distance from the readsensor by, e.g., a multiple of the axial length of the magnetized regionto be written. When the read sensor detects a damaged or erasedmagnetized region, the write head can be energized to restore thetransition zone to a proper degree of magnetization. Use of anintegrated write head enables magnetization of the piston rod duringfield service or maintenance of the machinery or equipment, withoutneeding an additional calibration device. The position-sensing systemcan employ a write head for each track (i.e., corresponding tomagnetized regions (FIG. 10A) or to sets 23 of read sensors (FIG. 3)).

Reliability

Various combinations of the above-described features operate to protectagainst accidental or intentional erasure of information recorded in themagnetically hard layer. One such feature is redundancy: read sensorssituated about the circumference of the piston rod read redundant axialtracks of magnetized regions or rings that encircle the piston rod.Thus, the entire circumference would need to be disturbed to causefailure of the position-sensing system. Another feature is the selectionof a high coercivity material, such as CoNiP, for the magnetically hardlayer, which can make the recorded information more difficult to erase.Perpendicular magnetization of the magnetically hard layer can furtherincrease the difficulty with which to demagnetize the magnetically hardlayer. In addition, improved GMR read sensors can read low amounts offlux and may therefore still be able to read incompletely erasedmagnetic bits.

Although the invention has been shown and described with reference tospecific preferred embodiments, it should be understood by those skilledin the art that various changes in form and detail may be made thereinwithout departing from the spirit and scope of the invention as definedby the following claims. For example, although the embodiments describedabove relate primarily to sensing piston rod position for linearmovement, the principles of the invention can be used to determinedistance traveled, position, velocity, acceleration, and movementdirection for sensing rotary motion of the piston rod with respect tothe cylinder. In addition, other embodiments of the invention canimplement the position-sensing system, including the magnetically hardlayer and read sensors, on components that are actuated by the pistonrod, instead of or in addition to being implemented on the piston roditself.

1. A position-sensing system, comprising: a cylinder; a piston rodmovably coupled to the cylinder for movement with respect thereto; amagnetically hard layer formed on the piston rod to provide a recordingmedium, a plurality of regions of the magnetically hard layer beingperpendicularly magnetized to provide an encoding scheme for determininga position of the piston rod relative to the cylinder; and a pluralityof magnetic-field sensors coupled to the cylinder in proximity of themagnetically hard layer to sense the perpendicularly magnetized regionswhile the piston rod is moving with respect to the cylinder, at leasttwo of the magnetic-field sensors being axially positioned to sense thesame set of magnetized regions in succession and to generate signals inresponse to the sensed magnetized regions that can be used to determinea position of the piston rod.
 2. The position-sensing system of claim 1,wherein the piston rod is made of a ferromagnetic material and themagnetically hard layer is disposed adjacent to an outer surface of theferromagnetic material without any intervening non-magnetic layer. 3.The position-sensing system of claim 1, further comprising amagnetically soft intermediate layer between the magnetically hard layerand the piston rod.
 4. The position-sensing system of claim 3, whereinthe piston rod is made of a ferromagnetic material.
 5. Theposition-sensing system of claim 3, wherein the piston rod is made of anon-magnetic material.
 6. The position-sensing system of claim 1,wherein at least one given magnetic field sensor is positioned at agreater distance from the magnetically hard layer than the magneticfield sensors positioned to sense the perpendicularly magnetizedregions, the given magnetic field sensor sensing an ambient field usedto perform common-mode rejection.
 7. The position-sensing system ofclaim 1, further comprising a power source supplying electrical power tothe plurality of magnetic field sensors and associated read-headelectronics when equipment using the position-sensing system is off sothat the magnetic field sensors can sense movement of the piston rodwith respect to the cylinder while the equipment is off.
 8. Theposition-sensing system of claim 1, wherein the plurality ofperpendicularly magnetized regions are disposed in spatially separatedtracks extending along in a direction of the motion of the piston rod,with at least two magnetic field sensors for each track ofperpendicularly magnetized regions.
 9. The position-sensing system ofclaim 1, wherein the piston rod is cylindrically shaped and theperpendicularly magnetized regions include rings around a circumferenceof the piston rod.
 10. The position-sensing system of claim 1, furthercomprising a flux concentrator near each magnetic field sensor of theplurality of magnetic field sensors, to improve sensing of theperpendicularly magnetized regions.
 11. The position-sensing system ofclaim 1, further comprising a second plurality of perpendicularlymagnetized regions for providing an absolute measurement associated withmovement of the piston rod.
 12. The position-sensing system of claim 1,further comprising a write head, integrated with the plurality ofmagnetic field sensors, for repairing damaged or erased perpendicularlymagnetized regions.
 13. The position-sensing system of claim 1, furthercomprising means for computing and storing positional information basedon output signals produced by the plurality of magnetic field sensors,wherein the positional information can be displayed or used to controlmovement or position of a machine or of a component thereof.
 14. Amethod for sensing a position of a piston rod moving relative to acylinder, the method comprising: forming a magnetically hard layer onthe piston rod to provide a recording medium for storing information;perpendicularly magnetizing information at a plurality of regions of themagnetically hard layer; reading, by a plurality of magnetic fieldsensors, the same perpendicularly magnetized regions of the magneticallyhard layer while the piston rod is moving relative to the cylinder; andgenerating output signals by the plurality of magnetic field sensorsfrom which a position of the piston rod with respect to the cylinder canbe determined.
 15. The method of claim 14, wherein the piston rod ismade of a ferromagnetic material and the magnetically hard layer isdisposed adjacent to an outer surface of the ferromagnetic materialwithout any intervening non-magnetic layer.
 16. The method of claim 14,further comprising providing a magnetically soft intermediate layerbetween the magnetically hard layer and the piston rod.
 17. The methodof claim 16, wherein the piston rod is made of a ferromagnetic material.18. The method of claim 16, wherein the piston rod is made of anon-magnetic material.
 19. The method of claim 14, further comprisingsensing an ambient field for use in performing common-mode rejection.20. The method of claim 14, further comprising supplying electricalpower to the plurality of magnetic field sensors and associatedread-head electronics when a machine integrated with the cylinder andpiston rod is off so that the magnetic field sensors can detect movementof the piston rod with respect to the cylinder while the machine is off.21. The method of claim 14, wherein the perpendicularly magnetizinginformation at a plurality of regions of the magnetically hard layerincludes perpendicularly magnetizing information in spatially separatedtracks extending in a direction of the motion of the piston rod, andfurther comprising reading each track of perpendicularly magnetizedinformation with at least two magnetic field sensors.
 22. The method ofclaim 14, further comprising concentrating flux near each of theplurality of magnetic field sensors to improve the reading of theperpendicularly magnetized regions.
 23. The method of claim 14, furthercomprising sensing a damaged or erased perpendicularly magnetized regionand dynamically repairing this region.
 24. The method of claim 14,further comprising computing and storing positional information based onoutput signals produced by the plurality of magnetic field sensors,wherein the positional information can be displayed or used to controlmovement or position of a machine or of a component thereof.